The catholyte which is also called alkaline water obtained by electrolyzing municipal water with a diaphragm-type electrolytic cell is effective as a medicine and is also said to have improved taste. Thus, the catholyte has enjoyed widespread use. Recently, the reduced quality of municipal water has resulted in an unpleasant odor and bad taste. As a countermeasure therefor, an apparatus for producing alkali ion water (alkaline water) has been widely used which is capable of simultaneously removing impurities and deodorizing the water by incorporating active carbon or a microfilter in the above-described electrolytic cell.
On the other hand, for the production and washing of electronic parts, specially prepared sulfuric acid, hydrofluoric acid, hydrogen peroxide, hydrochloric acid, etc., has hitherto been used. However, because impurities are introduced into the system and the purification technique for removing such impurities is troublesome, a method of producing acid water for washing by a water electrolysis technique has been proposed. The electrolysis is carried out by adding a slight amount of chloride ion to the anode chamber to thereby obtain an acid electrolyte having a very high oxidation reduction potential (ORP). Because the resulting solution has a strong sterilizing action and a strong disinfecting action initially as well as after use, sodium chloride or chloride ion alone remains to the same extent as in municipal water. Thus, when the used wash solution is discarded, problems such as secondary pollution, etc., do not occur. The above-described solution, therefore, has been widely used for various applications. However, as the case may be, there is a possibility of generating a small amount of organic chlorine compounds. Thus, one cannot conclude that such solutions are harmless to humans.
In the water electrolysis, when ammonium chloride (NH.sub.4 Cl) or sodium chloride, for example, is used as an electrolyte in the anode chamber, the anodic reaction is represented by: EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e.sup.-
The chlorine gas thus formed reacts with water to cause a disproportionation reaction represented by: EQU Cl.sub.2 +H.sub.2 O.fwdarw.2H.sup.+ +Cl.sup.- +ClO.sup.- +2e.sup.-
When the separation property of the diaphragm is sufficient, the solution in the anode chamber becomes acidic with the hydrogen ion of the hydrogen chloride thus formed, and a hypochlorous acid solution having a pH of 3 or less and an ORP of higher than 1.2 volts is formed. However, in practice, the separation property of the diaphragm is insufficient. When the electrolysis is carried out by adding a salt such as sodium chloride, etc., to the anode chamber for obtaining strong acid water, the ORP of the anolyte is increased but there is a problem in that the pH tends to not be sufficiently lowered.
To avoid this problem, there has been a proposal to increase the thickness of the above-described diaphragm and to increase the distance between the electrodes. This in turn would restrain diffusion of the liquids to thereby prevent the reaction products from mixing with one another. However, because the electric conductivity of water is quite low, a large electric current cannot be passed between the electrodes of the electrolytic cell and the actual current density is from 1 to 2 A/dm.sup.2. To obtain a large amount of acid water, plural electrodes must be combined. The resulting apparatus is large-sized which complicates the structure thereof including plumbing, and it takes too much time to maintain the apparatus.
To overcome these problems, the present inventors previously proposed a method of electrolysis using a cation-exchange membrane as the diaphragm, where the anode and cathode contact the respective sides of the cation-exchange membrane to form a substantial solid electrolyte. According to this method, when the current density is increased by a factor of 10 times or higher, that is, to 10 A/dm.sup.2 or higher, the cell voltage is maintained at a few volts. This makes electrolysis possible at a voltage far lower than that found in conventional methods. In this method, the present inventors also proposed a method of producing an acid water having a high ORP in the anode chamber by carrying out the electrolysis while adding a chlorine-containing compound such as hydrochloric acid, sodium chloride, etc., to the anode chamber.
In this method, acid water having a high ORP is obtained in the anode chamber, alkaline water having a low ORP is obtained in the cathode chamber, and both the anolyte and catholyte are useful for washing. The electrolytic reaction for obtaining acid water by this method is the formation of hypochlorite ion by the oxidation of chloride ion as described above. However, this method is disadvantageous in that the oxidation efficiency of the chloride ion is not always increased. Thus, a large amount of the above-described chlorine-containing compound had to be added to the anolyte. Furthermore, because the hydrogen ion thus formed permeates into the cathode chamber through the cation-exchange membrane, in order to achieve a sufficiently low pH, excessive electrolysis is required. Moreover, even when excessive electrolysis was carried out, a sufficiently low pH was not always obtained.
The ORP obtained by electrolysis of the above-described chlorine-containing compound is regulated by the concentration of the hypochlorite ion thus formed. Hypochlorite ion in a concentration of from 1 to 5 pm is sufficient, and chlorine gas is generated if the concentration thereof exceeds 5 ppm. If the current efficiency of chlorine generation is assumed to be 10%, the pH is from 1 to 4. To achieve a desired pH of 3 or lower, which is a measure of strong acid water, excessive electrolysis which ignores the current efficiency of chlorine generation is needed. Chlorine gas is generated when the chloride ion concentration is high and when the chloride ion concentration is low, the electrolysis amounts to simple water electrolysis.
To overcome these problems, the concentration of chloride ion in the anolyte present at the surface of the anode or in the vicinity thereof desirably is relatively low. If possible, this would be an ideal method of producing acid water such that almost all of the chloride ion is effectively used for forming hypochlorite ion and excessive residual chlorine is not present in the acid water thus formed. However, it was hitherto considered impracticable to maldistribute the chloride ion, which is dissolved and diffused in the whole anolyte, so that it has a higher concentration in the vicinity of the anode.